1. Field of the Invention
The present invention relates to an amplifier circuit including a transistor, and more particularly, to an amplifier circuit capable of preventing thermal runaway and optimizing response characteristics in a frequency band to suppress oscillations and remove harmonic components.
2. Description of the Related Art
An amplifier circuit, a circuit amplifying an input signal to generate an output signal, is widely used in electronic device power supply systems as well as in amplifying a communication signal of a wireless communication device. An amplifier circuit may include a bias circuit applying a bias signal for operating the amplifier circuit in a voltage or current form, in addition to an amplifier circuit amplifying an input signal to generate an output signal.
An amplifier circuit generally includes a plurality of elements such as an operational amplifier, a transistor, a resistor, and the like, and receives power required for driving from battery voltage, power supply voltage, or the like, input to the electronic device. In this case, in consideration of the characteristics of an electrical element, heat may be generated in the amplifier element during signal amplification. In the case in which the generated heat is not efficiently controlled, a thermal runaway phenomenon may occur, deteriorating the performance of, or even destroying, the amplifier element.
The thermal runaway phenomenon may be a serious problem, particularly in an amplifier circuit including a plurality of transistors. When a temperature of a transistor operating as a signal amplifier element within the amplifier circuit increases, a beta (β) value corresponding to a ratio between a collector current and a base current of the transistor decreases, and a larger base current is generally required in order to maintain current in a collector terminal connected to an output terminal. Therefore, in order to apply the larger base current thereto, voltage may be increased, whereby the thermal runaway phenomenon may occur.
In the following related art document, Patent Document 1 discloses an amplifier circuit including a plurality of transistors. The amplifying circuit includes a resistor connected between a direct current (DC) bias signal input terminal and a base terminal of the transistor in order to prevent a thermal runaway phenomenon. However, since only a capacitor and a resistor are connected to a signal input terminal RF_IN through which a wireless signal to be amplified is input, and the base terminal of the plurality of transistors, an effect of suppressing oscillations or transmitting only a signal having a specific frequency band may not be expected.